#24 / 118
24Cr51.996
Transition Metalssolid at STPD-block

Chromium

Group: 6Period: 4Standard Atomic Weight: 51.996 u

Why is Chromium in this position?

Understanding the scientific rationale behind Chromium's position in the periodic table:

Group Assignment
Group 6

Belongs to Group 6 because it has 6 valence electrons (3d⁵ 4s¹).

Period Assignment
Period 4

Belongs to Period 4 because its outermost electron occupies n=4.

Orbital Block
D-block

Belongs to the d-block because its valence electrons fill the 3d subshell.

Chemical Category
Transition Metals

Classified as a transition metal with notable catalytic utility and multiple oxidation states.

Atomic Structure & Bohr Shell Model

Shell Distribution: [2, 8, 13, 1]

Bohr Atomic Shell Model(2, 8, 13, 1)

Hover or tap any shell orbit ring to inspect electron counts and 2n² capacities.

24 Protons (p⁺)28 Neutrons (n⁰)
KShell K: Electron 1 of 2Shell K: Electron 2 of 2LShell L: Electron 1 of 8Shell L: Electron 2 of 8Shell L: Electron 3 of 8Shell L: Electron 4 of 8Shell L: Electron 5 of 8Shell L: Electron 6 of 8Shell L: Electron 7 of 8Shell L: Electron 8 of 8MShell M: Electron 1 of 13Shell M: Electron 2 of 13Shell M: Electron 3 of 13Shell M: Electron 4 of 13Shell M: Electron 5 of 13Shell M: Electron 6 of 13Shell M: Electron 7 of 13Shell M: Electron 8 of 13Shell M: Electron 9 of 13Shell M: Electron 10 of 13Shell M: Electron 11 of 13Shell M: Electron 12 of 13Shell M: Electron 13 of 13NShell N: Electron 1 of 1CrZ = 24

Educational Note: This Niels Bohr planetary model visually illustrates principal quantum energy shells ($n=1, 2, 3\dots$) and electron counts. In modern quantum mechanics (Schrödinger model), electrons do not orbit in fixed circular planetary tracks, but exist as 3D probability clouds (orbitals: $s, p, d, f$) governed by the Heisenberg uncertainty principle.

Electron Shell Filling Breakdown

Shell K (n=1):2 / 2 electrons (100%)
Shell L (n=2):8 / 8 electrons (100%)
Shell M (n=3):13 / 18 electrons (72%)
Shell N (n=4):1 / 32 electrons (3%)
Aufbau Electron Configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

Neutral ground state configuration. Valence electrons: 6.

Atomic & Quantum Properties

Electronegativity (Pauling)1.66 Pauling
1st Ionization Energy652.9 kJ/mol
Electron Affinity-64.3 kJ/mol
Atomic Radius (empirical)166 pm
Common Oxidation States+3, +6, +2
Crystal StructureBCC

Physical & Thermal Properties

Density at STP7.19 g/cm³
Melting Point1907 °C (2180 K)
Boiling Point2671 °C (2944 K)
Magnetic OrderingAntiferromagnetic
Discovery Year1797
Discovered ByLouis-Nicolas Vauquelin

Real-World Uses, Occurrence & Compounds

Major Industrial & Everyday Uses
  • Stainless steel alloy manufacturing (minimum 10.5% Cr prevents iron rusting)
  • Decorative and protective chrome electroplating on automotive bumpers and fixtures
  • Chrome tanning of animal hides into soft, water-resistant leather
  • Industrial dyes, pigments (Chrome yellow / PbCrO₄), and refractory bricks
Occurrence in Nature

Mined predominantly from chromite ore (FeCr₂O₄), with top deposits in South Africa, Kazakhstan, and India.

Etymology & Name Origin

From Greek 'chroma' meaning color, due to its multicolored chemical compounds

Important Chemical Compounds
Cr₂O₃ (Chromium oxide green)
K₂Cr₂O₇ (Potassium dichromate)
CrO₃ (Chromium trioxide)
PbCrO₄ (Chrome yellow)
Interesting Chemical Facts
  • Chromium is a classic periodic table anomaly: it adopts a 3d⁵ 4s¹ configuration instead of 3d⁴ 4s² because half-filled d-orbitals provide extra exchange energy stability.
  • The vivid red color of rubies and the deep green color of emeralds are both caused by trace chromium ions in the crystal lattice.
  • Elemental chromium is the only solid element that exhibits antiferromagnetic ordering at room temperature.
Safety & Handling Note

Trivalent chromium (Cr³⁺) is an essential trace nutrient, but hexavalent chromium (Cr⁶⁺) is a potent carcinogen, mutagent, and cellular toxin.